Program Director/Principal Investigator (Last, First, IVliddle): ROSSMANN, Michael G. PROJECT SUMr4ARY (See instructions): When this project started in the early 70's. the objective had been to determine the three-dimensional atomic resolution structure of a virus - any virus. With the attainment of this goal some eight years later, attention shifted from plant viruses to the greater challenge of animal viruses. The first success came in 1985 with the work on human rhinovirus serotype 14. The associated technological developments provided the momentum for structural studies of a large variety of animal, plant and bacterial viruses in the Principle Investigator's laboratory as well as elsewhere. The focus of these projects was the interaction of viruses with cellular factors such as receptors and neutralizing antibodies, the conformational changes that occur during cell infection, and the assembly of the virus from its constituent parts. Since the mid 90s there have been major advances in the use of cryo-electron microcopy and image reconstruction techniques for structural studies of uncrystallizable viral assemblies in combination with crystaliographic investigations of smaller viral components. This has made it possible to obtain pseudo-atomic resolution structures of enormous viruses with diameters ranging from 1000 to 7,500A and with genomes that provide the potential for most of the function of a biological cell. Many of these giant viruses retain at least approximate icosahedral symmetry both of their capsid and, often, of their internal lipid envelope, providing a handle for the investigation of particles that approach the size and complexity of a small bacterium. Currently we are studying three groups of large, icosahedral, dsDNA viruses: (i) the algal Paramecium bursaris chlorella virus-1 (PBCV-1). (ii) the insect Chito irredescent virus (CIV) and (iii) the amoeba Mimivirus. We have also initiated X-ray crystaliographic studies of the major capsid proteins of these viruses as well as specific viral proteins such as the unique glycosyltransferases that modify the viral surface proteins essential for virus assembly and interaction of the virus with its environment. At the same time the earlier work on small, icosahedral (ssRNA) picorna- and (ssDNA) parvoviruses has continued during the current project period as planned. We have studied the interaction of polioviruses, echovoviruses, and Coxsackievirus with their cellular receptors. Similarly, we studied the interaction of parvoviruses with their cellular receptors and with neutralizing antibodies. The emphasis over the next few years will be on the large icosahedral viruses, their organization and their interaction with their hosts, using cryo-electron tomography and electron microscopy of the whole virus and of thin sectioned virus, as well as crystallography of their component parts. RELEVANCE (See instructions): The large dsDNA viruses we are studying are approaching the complexity of a small bacterium. Thus, our current studies of large viruses are gradually merging with the study of small cells as we develop our technology. Although initially we benefitted by the presence of icosahedral symmetry present in these viruses but not in bacteria, we are now learning how to relinquish strict symmetry to find out more about these viruses and their interactions with cells.